When a Skeleton/Wither Skeleton attacks a target at melee, it will sometimes get stuck doing a bugged out animation where it quickly raises and lowers its Melee weapon (If it has one), and will only sometimes do damage this way. They also will sometimes go for a couple seconds where they simply ignore their target, even if they are damaging the skeleton. This is possibly two glitches, as there appears to be both a problem with pathfinding and the arm swinging glitch.
This only has an effect on 1.14 and 1.14.1, as it was NOT an issue in 1.13 (I tested it)
Sounds like MC-87093.